have an option in robyn_refresh to prevent window_start from sliding

[louisjanin]

Hello there,
In some cases, when first launching the MMM project, data points are sparse, data was not always meant to be kept or at the good time granularity for this kind of project.
When refreshing a model with new data, it can be interesting not to apply the initial modeling window but rather keep the window start fixed and only lengthen the window with the added data points?
I might miss something though, dont hesitate to correct me if refreshing should always keep the same window size...
A solution to manually change the window_start in the saved json file of the about-to-be-refreshed model was tested and seemed to work FYI. But an option in the robyn_refresh to either set the window_start or the number of periods would be easier to handle.
Thanks!